This application claims the benefit of Korean Patent Application No. 1999-40985, filed on Sep. 22, 1999, which is hereby incorporated by reference for all purposes as if fully set forth herein.
1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display panel, and more particularly to a method of driving a liquid crystal display panel that is adaptive for providing the entire panel with a uniform brightness.
2. Discussion of the Related Art
Generally, in a liquid crystal display panel, a liquid crystal layer controls a transmissivity of a light generated from a backlight in accordance with a voltage level of a data signal applied to the liquid crystal layer to display a picture. Such a liquid crystal display panel has a structure in which pixels provided with a liquid crystal layer and pixel electrodes and a reference electrode for applying a driving voltage to the liquid crystal layer and a reference electrode are arranged in a matrix type.
FIG. 1 is a schematic view of a liquid crystal display and a driving apparatus therefor. In FIG. 1, each pixel 22 is provided at each of intersections between m data lines D1 to Dm and n gate lines G1 to Gn within the liquid crystal panel 20. The pixels 22 arranged along each gate line form scanning lines and are connected, via the gate lines G1 to Gn, to a gate driver 24. Also, the pixels 22 are connected, via the data lines D1 to Dm, to the data driver 26. An equivalent circuit of the pixel 22 as a unit picture element is illustrated within an exploded view within the xe2x80x9ccirclexe2x80x9d of FIG. 1. Herein, a liquid crystal layer driven by a voltage difference between the pixel electrode and the reference electrode within a single pixel 22 is equivalent to a liquid crystal capacitor Clc. The pixel electrode is connected to a drain electrode of a thin film transistor (TFT) as a switching device, whereas the reference electrode is connected to a common voltage source Vcom. A gate electrode and a source electrode of the TFT are connected to a gate line and a data line, respectively.
The gate driver 24 sequentially applies a gate driving voltage to each gate line G1 to Gn to drive each scanning line of the panel sequentially. If a voltage is applied, via the gate lines G1 to Gn, to the gate electrodes of the TFT, then a channel is formed between the source electrode and the drain electrode of the TFT. At this time, a data voltage applied from the data driver 24, via the data lines D1 to Dn, to the source electrode of the TFT is applied to the drain electrode of the TFT. A difference voltage between a voltage applied to the drain electrode and a common voltage source Vcom is charged in the liquid crystal capacitor Clc to drive a liquid crystal layer of each pixel 22. Then, the liquid crystal layer controls a transmissivity of a light generated from the backlight in accordance with a difference voltage between the common voltage source Vcom and the data voltage.
In a general color display panel, a mixed ratio of colors three red (R), green (G) and blue (B), is controlled to realize various colors. In the liquid crystal display panel, red (R), green (G) and blue (B) color filers are mounted at each pixel 22 for transmitting a white light, or a color filer is replaced by three backlight lamps for generating red (R), green (G) and blue (B) lights. A driving method of a liquid crystal display panel without color filters is different from that of a liquid crystal display panel with color filters. In a liquid crystal display panel including three color backlight lamps instead of color filters, one frame making a picture is trisected to apply red (R), green (G) and blue (B) color data to the panel sequentially during each frame interval.
FIG. 2 is a timing chart showing an operation process made during one frame interval in a liquid crystal display panel with no conventional color filter. Referring to FIG. 2, in the case of the liquid crystal display panel with no color filter, a data voltage for each of the red (R), green (G) and blue (B) colors applied from the data driver 26 is time-divided during one frame interval to be sequentially charged in the pixels 22 of the panel 20. A backlight lamp having the corresponding color is turned on from a certain time when a data voltage for one color is being charged sequentially for one scanning line within the panel 20, until a time when a data voltage for another color begins to be charged in each ⅓ frame interval.
Herein, to turn on the backlight lamp having the corresponding color before a charge of a data voltage for any one color has been completed aims at lengthening a lamp turn-on time sufficiently to improve the brightness of a picture. If the backlight lamp is turned on before a data voltage for any one color was charged in all of the pixels 22 within the panel 20 as mentioned above, however, there exists a problem in that color purity of a picture displayed on the lower part of the panel 20 is deteriorated. As described earlier, during a time interval when a data voltage is charged in the panel 20, the gate driver 24 drives each gate line G1 to Gn in sequence from the first gate line G1 to the n gate line Gn. In other words, a scanning direction of the panel 20 is set to a direction going from the upper end of the panel to lower end thereof. In the pixels within the scanning line to which a gate voltage is applied, a conductive channel is provided between the source electrode and the drain electrode of the TFT to charge a data voltage applied, via the data driver 24, from the data lines D1 to Dm. Accordingly, if the backlight lamp is turned on before the scanning lines provided at the lower part of the panel 20 have been charged, then color purity of a picture displayed on the pixels at the lower part of the panel 20 is deteriorated because they is in a state of maintaining a data voltage for the preceding color. In order to solve this problem, the liquid crystal display panel with no color filter takes advantages of a scheme of simultaneously resetting all the pixels 22 within the panel 20 before applying a data voltage for any one color, to erase the entire previous data having been charged into each pixel 22 as shown in FIG. 2. If such a scheme is used, then, even though the backlight lamp having the corresponding color is turned on before charging of a data voltage for any one color has been completed, the pixels in which charging of the data voltage for the color has not been made go into a state of erasing the data for the preceding color, so that it is possible to prevent a problem of the color purity deterioration caused by residual data.
In a driving method including the step of sequentially charging a data voltage and the step of simultaneously resetting the pixels 22, however, a brightness non-uniformity phenomenon, differentiating the brightness of a picture displayed on the upper part of the panel 20 from the brightness of a picture displayed on the lower part thereof, is generated. Such a problem will be described in conjunction with FIG. 3 and FIG. 4. In the conventional panel driving method, each gate line G1 to Gn provided within the panel 20 is driven in sequence from the first gate line G1 positioned at the top of the panel, to the nth gate line Gn positioned at the bottom thereof. As shown in FIG. 3, the scanning direction of the panel 20 is always constant for each frame interval. As mentioned above, when all the pixels 22 are simultaneously reset prior to charging the next data, a data sustaining interval until a pixel 22 is to be reset becomes different in accordance with whether the pixel 22 is located at any part of the panel. In other words, since all the pixels 22 are not charged simultaneously, the data-sustaining intervals of the pixels 22 become different for each scanning line at the reset time. For instance, data sustaining intervals between A pixels positioned at the first scanning line of the panel 20, B pixels positioned at the middle scanning line of the panel 20 and C pixels positioned at the nth scanning line at the bottom of the panel 20 as shown in FIG. 1 become different as shown in FIG. 4. A data sustaining interval of the A pixels in which a data voltage is first charged is longest, whereas a data sustaining interval of the C pixels in which a data voltage is last charged is shortest. As described above, the backlight lamp is turned on after a data charge for all the pixels 22 has been completed, but it is turned off after being turned on in the course of a scanning interval of the panel 20 prior to a reset interval of the next pixels 22 so as to improve the brightness. Accordingly, turn-on intervals of the A pixels, the B pixels and the C pixels become different, and a difference in turn-on interval is always generated every frame when a scanning direction of the panel 20 is always constant every frame to cause a brightness difference between the upper part and the lower part of the panel 20.
Such a problem also is generated in the case of driving a liquid crystal display panel with color filters. In a liquid crystal display panel mounted with a color filter for each pixel and including a single backlight lamp, red (R), green (G) and blue (B) data are simultaneously applied every frame as shown in FIG. 5. Also, a scanning direction of the panel 30 is always constant from the upper end of the panel 30 until the lower end thereof. The liquid crystal display panel 30 with color filers provides a data reset interval for each frame so as to prevent a phenomenon of leaving an image from the previous frame onto a residual image when a picture is changed frame by frame to exhibit a slow response speed. The problem related with the residual image is solved by eliminating during the reset interval data which was charged into each pixel in the previous frame. In such a case, the sustaining interval of a data voltage charged into the pixel becomes different in accordance with a position of the pixel within the panel 30 as shown in FIG. 4. Accordingly, since a difference in a data turn-on interval according to a position of the pixel is always generated every frame when a scanning direction of the panel 30 is always constant for each frame, a brightness non-uniformity phenomenon according to a position of the pixel is generated at the panel 30.
Accordingly, the present invention is directed to method of driving liquid crystal display that substantially obviates one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method of driving a liquid crystal display panel that is capable of providing the panel with an entirely uniform brightness.
Additional features and advantages of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.